The invention relates to control of clip carriages in simultaneous biaxial tenters for stretching webs of material wherein clip carriages are independently propelled in two opposed endless loops. The invention in particular relates to tenters using linear motors to propel the carriages throughout the endless loops. On the inner facing sides of the two loops, the web is grasped by clips on the carriages at the entrance of each loop and stretched longitudinally by progressively accelerating individual carriages thereby causing them to become spaced apart. At the same time the carriages can be guided laterally apart thereby stretching the web laterally. At the end of each loop, the web, now traveling at a higher speed than at the entrance, is released from the carriages and the carriages are returned to the beginning of the loop where the web is once again grasped and stretched by the diverging accelerating carriages. On the return side of each loop, the carriages must be slowed from the web release speed to the web grasping speed, at which speed the carriages are abutted with each other. Such a linear motor propelled, web tenter system with the above carriage control is described in U.S. Pat. No. 5,072,493 to Hommes et al. Additional details and improvements in such system are described in patent publications DE 4436676 to Steffl (clip carriages), DE 19513301 to Briel et al (eddy current disc for carriages stacking), and DE 19517339 to Ruehlemann (rail transitions at exit turn).
Controlling the carriages on the return side of each loop has presented challenges in handling the carriages during start up of the tenter and process upsets, handling large gaps between carriages that may cause a shortage of carriages at the web grasping end of the loop, and handling the same number of carriages at different stretching ratios. At a low stretch ratio, a lot of carriages are on the web sides of the loops and fewer are on the return sides, while for high draw ratios, fewer carriages are on the web sides of the loops and more are on the return sides. There are also economic concerns about keeping the return side system simple and low cost since precise control of the carriages is not required on the return side where there is no interaction with the web.
Nevertheless, one suggested method of operating the return side is to provide linear motors that interact synchronously with permanent magnets on the carriages to predictably control the speed and position of the carriages. These magnets are the same magnets that are required for precise control on the web stretching side of the loops. For precise return side control, the motor drives for control of adjacent motor windings can be frequency and phase synchronized or partially frequency and phase synchronized as the carriages pass from one motor winding to the next. For less precise control and lower cost, the motor drives may not be frequency and phase synchronized as the carriages pass from one motor winding to the next; some loss of control of speed and position will be experienced, but it may not affect overall operation.
At the loop turns at the web entrance ends of the endless loops, an eddy current disc, as described in the '301 reference, may be employed for each loop. It is driven at a speed faster than the desired carriage speed at the web entrance of the loop. This removes all gaps between carriages and abutts them with a low collision force that does not damage the carriages, which are provided with bumpers for this purpose. The eddy current disc also develops sufficient cumulative force on the stack of carriages to insure that the carriages entering the film side of the machine are pushed tightly against the first few film side carriages with no gaps. This pushing force also contributes to the force required of the first few carriages on the film side to develop web tension prior to the tenter.
Although this improved system achieves a simplified operation and reduced costs, there is a problem that this system does not handle large gaps that may occur between carriages unpredictably. For instance, on the web side a gap between carriages may occur when there is a web break during thread-up or continuous operation that causes the carriages to get out of synchronism with the electromagnetic wave developed by the motor winding. In this situation, one or several carriages may fall behind and bunch-up leaving a large upset gap ahead of the bunched-up carriages. For the bunched-up carriages to catch up to their steady state positions, the upset gap must be closed before it reaches the entrance end of the tenter. When this gap gets to the return side, the only way proposed by known systems to close the gap is by increasing the speed of the eddy current disc, but this has been found to be insufficient for closing a gap greater than about 2 meters, and cannot handle multiple, or recurring, closely spaced gaps of 1.5-2.0 meters. The eddy current disc is torque controlled so its speed increases over a limited range when a gap is experienced by the disk. This is because the resisting force the carriages exert on the disk decreases with fewer carriages adjacent the disk. Since the disk is running at a constant torque, the lower resistance causes the disk to speed up. As the disk speeds up, the relative motion between the disk and carriages stacked adjacent the disk increases until the resisting force of the carriages and the driving torque of the disk is again in balance. The increased speed of the disk allows carriages that are up to two meters behind to catch up to the stack of carriages on the disk. With a gap greater than two meters, the bunched-up carriages arrive too late at the entrance end and a gap occurs on the web side of the loops which shuts down the tenter. Additionally, there may be so few carriages engaged by the eddy current disc that there is insufficient force developed to achieve the necessary web tension by the first few carriages on the web sides of the loops. Shut down of the tenter is highly undesirable for a tenter that stretches continuously cast polymer sheet that must be diverted to waste during resynchronization of the carriages. There is a need for a simple system for handling large gaps (greater than 2 meters) or frequently recurring gaps of 1.5-2.0 meters to maintain a continuous supply of abutted carriages to the eddy current disc and the web side of the loops.